1. Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Background Art
Silicon carbide (SiC) enables manufacturing of a silicon carbide semiconductor device having higher breakdown voltage characteristics as compared with silicon (Si) which has been conventionally used, and is expected to provide a high-power semiconductor device for the next generation. In manufacturing a silicon carbide semiconductor device using such silicon carbide, to control a conductive type and conductivity, an n-type or p-type impurity ion is implanted in a silicon carbide wafer having a silicon carbide layer formed on a silicon carbide substrate by epitaxial growth, and after the ion implantation, in order to activate the implanted ion and additionally recover a crystal defect formed by the ion implantation, an annealing process step is performed in which the ion-implanted silicon carbide wafer is exposed to a high temperature in an inert gas atmosphere such as an argon (Ar) atmosphere. In a case where a silicon carbide wafer is used, in order to stabilize characteristics, it is preferable that the annealing process is performed under a temperature as high as possible, normally 1500° C. or higher, and desirably 1600° C. or higher.
However, when a silicon carbide wafer is annealed at a high temperature, a surface roughness called step bunching occurs in a surface of the silicon carbide wafer. The reason why the step bunching is formed is as follows.
A silicon carbide wafer is normally obtained by forming a silicon carbide layer on a silicon carbide substrate by epitaxial crystal growth. In this epitaxial growth, a growing crystal axis is inclined by 4 or 8 degrees with respect to a c-axis direction (a direction perpendicular to the [0001] plane which is a crystal plane), in order to prevent different crystal forms such as the 6H-1 type and the 4H type from occurring in the same crystal plane.
When a silicon carbide wafer obtained by the crystal growth with the crystal axis being inclined in this manner is exposed to a high temperature in the annealing process for example, Si and carbon (C) which are constituent elements evaporate from a surface of the silicon carbide wafer. In this evaporation, since silicon and carbon evaporate under different evaporation conditions and moreover the crystal axis is inclined, the amount of evaporation of silicon and the amount of evaporation of carbon differ in a silicon carbide wafer plane, which consequently causes the step bunching on the surface of the silicon carbide wafer.
The step bunching thus formed becomes an obstacle to the formation of a gate oxide film on the silicon carbide wafer after the annealing process, and also becomes an obstacle to the formation of a gate electrode on the gate oxide film. For example, there is a possibility that adhesion and leakage characteristics may deteriorate because a boundary surface between the silicon carbide wafer and the gate oxide film or between the gate oxide film and the gate electrode is uneven.
Therefore, preventing or reducing the step bunching is a significant problem in stabilizing the quality of and improve the yield of the silicon carbide semiconductor device.
As a method of preventing or reducing the step bunching, there is a method in which a carbon film is formed on a surface of a silicon carbide wafer and used as a protective film for preventing evaporation of silicon and carbon in the annealing process.
Japanese Patent Application Laid-Open No. 2009-65112 discloses a method of forming a carbon film using a hydrocarbon material gas or a hydrocarbon gas containing oxygen such as alcohol.
When a carbon film is used as a protective film for preventing evaporation of silicon and carbon in an annealing process, a certain degree of thickness is required. Thus, a film thickness control is an important factor, and it is necessary to examine whether a formed carbon film has a desired thickness or not.
To accurately measure the thickness of a carbon film, it is necessary that a silicon carbide wafer after the formation of the carbon film is cut in a thickness direction and the film thickness is measured on a resulting cross section. However, a silicon carbide wafer is very expensive, and cutting it for the purpose of measuring the thickness of the carbon film involves an problem that a manufacturing cost of a semiconductor device is increased.